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Stellar Evolution
The Birthplace of Stars
The space between the stars is not completely empty. Thin clouds of hydrogen
and helium, seeded with the “dust” from dying stars, form in interstellar space.
Dark Clouds
gather
Molecular Clouds
Sometimes (especially in spiral arms), the gas is compressed enough that the dust is thick
and gravity can collapse knots in these “molecular” clouds to make new stars.
Formation of stars
• First what is formed is a protostar
• Protostars with mass 0.08MS do not develop the pressure and
temperature necessary to initiate nuclear fusion reaction and will
contract to become a brown dwarf.
• Protostars with mass greater than 100MS develop a pressure a
pressure much higher than the gravitational pressure that disrupts
the evolution of the star
• When the star is formed, it joins the main sequence the star, it
location depending on the mass of the star.
Evolution on the
Main Sequence (I)
Main-Sequence
stars live by
fusing H to He.
Zero-Age
Main
Sequence
(ZAMS)
MS evolution
Finite supply of H
=> finite life time.
Evolution on the
Main Sequence (II)
Evolution off the Main Sequence:
Expansion into a Red Giant
• H in the core completely
converted into He – radiation
reduces and core begins to
collapse due to gravity
resulting in an increase in
temperature
• This leads to “H burning” in
the outer shell around the
core
• Thus the core continues
shrink while the outer shell
expands and cools
• This is the RED GIANT
phase
Expansion onto the Giant Branch
Expansion and
surface cooling
during the phase of
an inactive He core
and a H- burning
shell
Sun will expand
beyond Earth’s orbit!
Red Giant Evolution
He-core gets denser
and hotter until the next
stage of nuclear burning
can begin in the core:
4 H → He
He fusion:
He
3 4He → 12C
“Triple-Alpha Process”
Fusion of Helium into
Carbon
This is followed by
4He
+ 12C → 16O
Evolution after Red Giant Phase
– Low mass stars
• For mass less than 4MS the star becomes unstable. The star loses
the outer envelope of the star of gases exposing the inner core of
oxygen and carbon  Galactic nebula. Eventually this core cools to
become white dwarf
• If the mass of white dwarf is less than 1.4MS , it is able to support
itself due to electron degeneracy pressure and remain stable
• When the white dwarf mass exceeds this limit (Chandrasekhar’s
limit), then it collapses further due to gravity to become neutron star
Evolution after Red Giant Phase
– High mass stars
•
•
For mass greater than 4MS , fusion in the core continues resulting in the
formation of Ne, Si, and finally Fe.
No more thermonuclear reaction happen, and gravity takes over collapsing
the core. This collapse is an IMPLOSION termed as the type II Supernova
Evolution after Red Giant Phase
– High mass stars
•
During the Supernova collapse, the protons and neutrons are crushed to
form neutrons. Eventually the remnant of this collapse is a neutron star
•
When the mass of the neutron star exceeds 2-3 times the mass of Sun (this
limit is not precisely estimated), neutron degeneracy pressure does not
allow stability. The neutron star collapses further to become a Black hole.
This is known as the Oppenheimer-Volkoff limit
Summary of Post-Main-Sequence
Evolution of Stars
Fusion proceeds to
formation of Fe core.
M > 8 Msun
Fusion
stops at
formation
of C,O
core.
M < 4 Msun
M < 0.4 Msun
Evolution of
4 - 8 Msun
stars is still
uncertain.
Red dwarfs:
He burning
never
ignites
High-mass stars evolve off the main
sequence (to become red giants)
earlier than low-mass stars.
=> For a given age, low-mass stars
are still on the MS, while high-mass
stars are already red giants!
Red shift of light from galaxies
•
•
Due to expansion of universe, galaxies move away from each other.
This leads to red shift in the light received from these galaxies
(Doppler Effect)
Red shift formula
The relativistic red shift formula is
(z is red shift parameter)
1  v 


 f
c

 1
z



f
1  v 


o
o
 c
At low speeds
1  v 


 f
c

 1  v
z



f
c
1  v 


o
o
 c
Red shift formula - problem
Estimate the speed of a galaxy, if the wavelength for the hydrogen
line at 434nm is measured on earth as being 610nm.

v
z


c
o
v
c

o
3  108  176

 1.21 108 ms  1
434
Hubble’s law
"The distance to objects beyond the Local Group is closely related to
how fast they seem to be receding from us"
v  Hd
“ v ” is the recessional speed of the
galaxy, “ d” is the distance of the
galaxy from us and “ H” is the Hubble
parameter or Hubble constant.
H = 71 km/s/Mpc = 22km/s/Mly
Hubble’s law - limitations
•
•
•
Can be applied to galaxies other than local cluster
Galaxies in local cluster may even show blue shift (Andromeda moves
towards Milky Way)
Distance and speed of distant galaxies cannot be accurately estimated –
this lead to uncertainties in the estimation of Hubble’s constant
Measurement of Hubble constant
• Observing Cepheid variables in different galaxies
• Observing Supernova explosions
(READ FURTHER TO COMPLETE THIS TOPIC)
Age of the universe
• Reciprocal of Hubble constant gives a rough estimate of the age of
the universe (Here we make a assumption that H is really a
constant)
• Rough estimate of the age of universe is 10 to 20 billion years
Sample problem based on Hubble law
Solution to problem in previous slide
Smile please ….
Astrophysics class is FINALLY OVER ….